A process for the preparation of carbon layers by pyrolysis of organic compounds or polymers on powdered ceramic supports is known. “A new method of coating powdered supports with conductive carbon films” (M. Molenda, R. Dziembaj, Z. Piwowarska, M. Drozdek, J. Therm. Anal. Cal., 88, 2007, 503-506) discloses a process for the preparation of conductive carbon layers by pyrolysis of polyacrylonitrile deposited on Al2O3 grains. According to the process, radical polymerization of acrylonitrile in an aqueous Al2O3 suspension was carried out, followed by controlled pyrolysis of the such-obtained precursor, under argon atmosphere, at 400-800° C. for 12 h. Similarly, pyrolysis of poly(N-vinylformamide) (PNVF) deposited on powdered Al2O3 by wet impregnation from an aqueous solution was performed in “Direct Preparation of Conductive Carbon Layer (CCL) on Alumina as a Model System for Direct Preparation of Carbon Coated Particles of the Composite Li-Ion Electrodes” (M. Molenda, R. Dziembaj, M. Drozdek, E. Podstawka, L. M. Proniewicz, Solid State Ionics 179, 2008, 197-201).
In the case of conductive carbon layers intended for lithium batteries, the layers should be characterized by sufficient electrical conductivity, as well as easy transport of lithium ions through the layer during charging and discharging. At the same time, they should exhibit possibly low contact surface area with an electrolyte to suppress formation of a SEI (Solid-Electrolite Interphase) layer. Electrical properties of a carbon layer are improved with increase of pyrolysis temperature, however given limited thermal stability of electrode materials in reductive conditions it is required that the pyrolysis process is carried out at the lowest temperature possible. Carbon layers obtained by pyrolysis of a PNVF precursor did not exhibit sufficiently high electrical conductivity and possessed a high specific surface area.
The process for the preparation of carbon layers on powdered supports comprises dissolving hydrophilic polymer (PH) at a level of 85 to 99.9% by weight in water, adding pyromellitic acid (PMA) or pyromellitic dianhydride (PMDA) at a level of 0.1-15% by weight, followed by adding to the mixture a powdered support at a level of 1-99% by weight. The suspension is concentrated and dried, and then the prepared composite precursor is subjected to pyrolysis at 300-1500° C.
Preferably, the hydrophilic polymer is poly(N-vinylformamide), polyacrylamide or N-vinylformamide-acrylamide copolymers
Preferably, the powdered support comprises metal oxides, lithium and transition metal silicates and polysilicates, lithium and transition metal phosphates and polyphosphates, germanates, vanadates, metals and metal alloys, metal nitrides and silicon.
Most preferably, the powdered support comprises Al2O3, LiMn2O4, Li1-xMn2-2xO4 (0≦x≦0.33), LiMn2O4-ySy (0≦y≦0.25), LiFePO4, LiMxFe1-xPO4 (M=V, Mn, Co, Ni, Cu; 0≦x≦1), Li2MSiO4 (M=V, Mn, Fe, Co, Ni, Cu), LiMSiO4 (M=V, Mn, Fe, Co, Ni, Cu), LiCoO2, LiMxCO1-xO2 (M=V, Fe, Co, Ni, Cu; 0≦x≦1), LiMn1/3CO1/3Ni1/3O2, Sn, SnO, SnO2, tin alloys, Si.
Preferably, the pyrolysis is carried out in inert conditions, preferably under inert gas atmosphere, most preferably argon, nitrogen or helium atmosphere.
Preferably, the pyrolysis is carried out in weakly reducing conditions.
Preferably, the pyrolysis is carried out under vacuum.
In the process of the invention, it was unexpectedly found that adding pyromellitic acid (PMA) or pyromellitic dianhydride (PMDA) to the hydrophilic polymer composition significantly enhances electrical properties of prepared carbon layers and at the same time allows decreasing temperature of the pyrolysis process.